"""
102. Binary Tree Level Order Traversal
Difficulty: Medium
https://leetcode.com/problems/binary-tree-level-order-traversal/

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Given the root of a binary tree, return the level order traversal of
its nodes' values. (i.e., from left to right, level by level).



Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []



Constraints:

        • The number of nodes in the tree is in the range [0, 2000].

        • -1000 <= Node.val <= 1000
"""

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
from collections import deque


class Solution:
    # T: O(n) S: O(n)
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        res = []

        q = deque()
        q.append(root)

        while q:
            qLen = len(q)
            level = []
            for i in range(qLen):
                node = q.popleft()
                if node:
                    level.append(node.val)
                    q.append(node.left)
                    q.append(node.right)
            if level:
                res.append(level)

        return res